1. Field of the Invention
The present invention relates to an electromagnetic actuator that linearly travels in an axial direction and, more particularly, to a moving-magnet type electromagnetic actuator that has a stator yoke on its outer peripheral portion and includes therein a movable section composed of one or more exciting coils, permanent magnets, and yokes, and also to a composite electromagnetic actuator apparatus.
2. Description of Related Art
An example of conventionally known electromagnetic actuators is a moving-coil type actuator that is used to drive an information read/write head of an information storage device, and adapted to directly drive the head linearly or rotationally and to position it to an appropriate track of a recording medium, thereby reading or writing information from or to the recording medium. This actuator, known as a voice coil motor (VCM), drives a head attached to a coil by making use of an electromagnetic force generated according to Fleming's left-hand rule, that is by causing current to flow through a coil that constitutes a component at right angles to a magnetic field. This type of actuator is capable of accurate positioning control by employing a feedback control technique within a linear range of a travel distance of about 10 mm or a rotational range of a rotation angle of about 90 degrees.
Another example of the electromagnetic actuators employs an inexpensive two-phase claw-pole stepping motor. In this type of actuator, a lead screw is formed on a motor shaft, and a head movably attached on the shaft through the screw moves linearly as the motor runs.
The moving-coil type (VCM type) actuator described above, however, has the following disadvantages:
(1) The travel range is large, so that the air gap length between a magnet and a coil cannot be set to a small value. This means that the magnetic flux density of the air gap cannot be set to a high value.
(2) A sufficient thrust or electromagnetic force cannot be obtained unless a high-performance magnet is used.
(3) The coil is movable, making it difficult to increase the number of turns. This inevitably leads to an increased bulk.
(4) Electric power must be supplied to the movable coil, requiring an expensive feeder harness.
(5) Since the travel range is large, supposing the mass of the movable section remains unchanged, equivalent frequency responsiveness cannot be secured unless a larger thrust is generated.
(6) The VCM cannot provide a magnetic circuit with a closed structure, resulting in large leakage flux to the outside.
(7) Since the leakage flux cannot be reduced, the use with a magnetic storage device may adversely affect its read/write reliability.
The above disadvantages have been placing major restrictions on using the actuator with a magnetic recording apparatus. In addition, there has been a problem that the cost cannot be reduced due to the shortcomings mentioned above.
On the other hand, an actuator employing a two-phase claw-pole stepping motor has the following disadvantages:
(1) A mechanical converting means such as a screw for converting a rotational movement into a linear movement is required.
(2) Performance of both high speed and high resolution is limited because the actuator does not employ a direct coupling method.
(3) A stepping motor based on an open-loop control is used as a driving source and hence, it is impossible to continuously perform positioning, and resolution of positioning is limited. In particular, current resolution available at present is about 100 μm at the best.
(4) This type of actuator generally employs an open-loop control, and is not suited for a closed-loop control.